The Aim of this proposal is to understand the function of one of the major vaccinia virus (VV) interferon (IFN)-resistance and neurovirulence genes, E3L. Since VV is the vaccine for smallpox, a potential biowarfare/bioterrorism agent, analysis of this virulence factor may lead to development of safer, more effective vaccines for defense against bioterrorism attacks. In addition, these safer, more effective strains of VV and all strains of variola virus, the causative agent of smallpox, this work may lead to development of anti-smallpox drugs. The work described in this proposal will continue investigations into defining the roles that the biochemical characteristics of the E3L-encoded proteins play in evasion of the host defenses by VV. Mutants that separately affect each of the known biochemical characteristics associated with E3L-encoded proteins will be prepared and characterized. These well characterized mutants will then be used to determine the role of each of the biochemical characteristics of E3L in each of the known biological functions of E3L, including pathogenesis in the mouse model. This system is unique in allowing analysis of the function of this important virulence gene from the molecular level to the level of pathogenesis in a whole animal. Thus, this work will lead to translation of basic molecular knowledge of E3L function into clinically relevant applications.